1. Field of the Invention
The present invention relates to a nonlinear-optical material and a nonlinear-optical device applicable to fields such as optical communication, optical wiring, optical information processing, sensors, and image processing, and a stock solution for producing the same.
More specifically, the invention relates to a nonlinear-optical device such as an optical switching device, optical modulating device, wavelength-converting device, or phase-shifting devices utilizing second-order nonlinear-optical effects, or a memory device or image-processing device utilizing photorefractive effects; and a nonlinear-optical material applicable to these applications; and a stock solution for manufacturing the same.
2. Description of the Related Art
Most of the functional devices important in optical fields are realized by the use of nonlinear-optical materials, in particular, second-order nonlinear-optical materials. Such devices include wavelength-converting devices, optical modulating devices, and optical switching devices, and these are important in optical fields such as optical communication, optical wiring, optical information processing, sensors, and image processing. Inorganic nonlinear-optical materials, such as lithium niobate, and potassium dihydrogen phosphate, have already been commercialized and are widely used as second-order nonlinear-optical materials, but recently, organic nonlinear-optical materials have been attracting attention due to advantages in their improved nonlinear-optical characteristics, lower material and production cost, higher productivity and the like. Extensive research and development is currently directed to commercialization of these materials in order to replace conventional inorganic materials.
An essential requirement in order to achieve the second-order nonlinear-optical effect is that there be, in principle, no center of symmetry in the system. Second-order nonlinear-optical materials are roughly classified into two systems, the first being a system where an organic compound exhibiting nonlinear-optical activity is crystallized in a crystal structure having no symmetry center (hereinafter, referred to as a “crystalline system”). The second system involves an organic compound exhibiting nonlinear-optical activity (i.e., a nonlinear-optically active organic compound) dispersed in a polymer binder and the symmetry center of the system is eliminated by orientation using a certain means (hereinafter, referred to as a “dispersion system”).
Although organic nonlinear-optical materials of the crystalline system are known to exhibit extremely superior nonlinear-optical characteristics, it is almost impossible to control the crystal structure artificially, and thus it is very rare that crystal structures without any symmetry centers are obtained. Even if obtained, it is difficult to produce the large organic crystals required for producing optical devices. In addition, these optical materials often cause various problems, as the organic crystals are very fragile or brittle and are often damaged during production of the devices.
In contrast, organic nonlinear-optical materials of the dispersion system are regarded as more promising, because they have greater potential for commercialization. This is due to the fact that they are provided with favorable characteristics such as good coatability and mechanical strength, attributes that are useful for producing devices with polymer binders.
Dispersion-type organic nonlinear-optical materials must be optically homogeneous and transparent, and thus, the nonlinear-optically active organic compound contained therein should be dispersed in the polymer binder uniformly without aggregation. In addition, as described above, it is necessary to orient the nonlinear-optically active organic compound by using a certain means in order to cause the anisotropy required for the second-order nonlinear-optical effect. It is also necessary order to obtain effective use of the functional devices and to preserve the oriented state consistently in hot or humid environments during production, operation, and storage of the resulting devices.
Accordingly, the nonlinear-optically active organic compounds to be used in dispersion-type organic nonlinear-optical materials must not only have superior nonlinear-optical characteristics, but also a lower tendency toward aggregation and higher compatibility with the polymer binder. The dispersion-type organic nonlinear-optical materials are generally converted to various devices in the form of thin films, and wet coating methods are favorably used for forming the thin films. For this reason, nonlinear-optically active organic compounds used in dispersion-type organic nonlinear-optical materials should be highly soluble in the stock solution solvents. It is also necessary for the polymer binders to have, in addition to qualities such as superior coatability and mechanical strength, a high glass-transition temperature in order to consistently maintain the orientation of the nonlinear-optically active organic compound contained therein.
It is necessary to orient the nonlinear-optically active organic compound as described above for generating the second-order nonlinear-optical activity in dispersion-type organic nonlinear-optical materials, and the electric field-poling method is commonly used as the orientation method. The electric field-poling method is an orientation method of applying an electric field to a nonlinear-optical material and orienting the nonlinear-optically active compound therein in the direction of the applied electric field by the Coulomb force between the dipole moment of the nonlinear-optically active compound and the applied electric field. The orientation of the nonlinear-optically active compound is generally assisted by the activation of molecular motion by heating to a temperature close to the glass-transition temperature during application of the electric field.
It is known that so-called push-pull π-conjugated compounds having an electron-donating group at one end of the π-conjugated chain and an electron-withdrawing group at the other end are effective nonlinear-optically active organic compounds. For example, Disperse Red 1 (generally, abbreviated to DR1) having an N-ethyl-N-(2-hydroxyethyl)amino group as the electron-donating group at the 4 position and a nitro group as the electron-withdrawing group at the 4′ position of the azo benzene structure (π-conjugated chain) is widely known as a typical nonlinear-optically active organic compound. However, given that DR1 does not in essence possess superior nonlinear-optical characteristics and has both a lower compatibility with ordinary polymer binders and a higher tendency to sublimate, DR1 is problematic in that it disappears with the application of heat in the drying and electric-field poling processes, and dialkylamino groups tend to oxidize and deteriorate.
Various nonlinear-optically active organic compounds have been developed hitherto to solve these problems, but compounds satisfying all of the required characteristics at the same time have yet to discovered. In particular, it is quite difficult to provide a compound superior having both nonlinear-optical characteristics and higher binder compatibility. Namely, the nonlinear-optical characteristics of push-pull π-conjugated compounds are known to be improved by generally elongating the π-conjugated chain therein and strengthening the electron-withdrawing capacity of electron-withdrawing group and the electron-donating capacity of electron-donating group. Nonetheless, the improvement in nonlinear-optical characteristics is accompanied by an increase in aggregation between molecules, consequently leading to a decrease in the compatibility of the conjugated compounds with the polymer binder.
For example, it has been disclosed that a compound having the structure shown below exhibited extremely superior nonlinear-optical characteristics, however, it is very difficult to produce a film where the compound is uniformly dispersed in the polymer binder and crystal precipitation is suppressed due to its extremely high tendency to coagulate (see e.g., Chemistry of Materials, 2001, Vol. 13, pp. 3043 to 3050). It has been also disclosed therein that it is necessary to use a halogenated solvent having a low boiling point as the coating solvent, but the use of such a halogenated solvent is not favorable as it negatively affects air quality.

On the other hand, although polymethyl methacrylate (generally abbreviated to PMMA) has been most intensively studied as the polymer binder, the glass-transition temperature of PMMA is lower at about 100° C., and thus the orientation of a dispersion-type organic nonlinear-optical material in the PMMA polymer binder gradually slackens even at room temperature. Nonlinear-optical materials derived therefrom exhibit a marked decrease in nonlinear-optical characteristics over time. Thus, PMMA-based optical materials are not suitable for actual use in functional devices (e.g., Chemical Reviews, 1994, Vol. 94, No. 1, pp. 31 to 75).
In order to solve these problems, binder polymers for replacing PMMA have been intensively studied, leading to polymers having glass-transition temperatures that are higher than that of PMMA. Examples of reported binder polymers include polycarbonate, polyimide, polysulfone, and polycyclic olefin (see e.g., Japanese Patent Application Laid-Open (JP-A) No. 6-202177). Use of these polymer binders having higher glass-transition temperatures is inevitably accompanied with an increase in the heating temperature required for electric-field poling, and this in turn causes oxidation and disappearance by sublimation of the nonlinear-optically active organic compounds such as DR1 during the electric-field poling process. The compatibility between these polymer binders having high glass-transition temperatures and a nonlinear-optically active organic compound such as DR 1 is not always high, and accordingly, addition of the nonlinear-optically active organic compound at a higher concentration for the purpose of improving the nonlinear-optical characteristics causes aggregation or crystallization of the compound. Further, addition of nonlinear-optically active organic compounds even at a lower concentration, still causes aggregation or crystallization by heating or the passage of time.
As a means for solving the problems of the dispersion-type organic nonlinear-optical materials described above, the introduction of a nonlinear-optically active organic compound to the main chain and/or side-chain of a polymer, i.e., conversion of the nonlinear-optically active organic compound to a polymeric compound, is being studied.
For example, a nonlinear-optically active polymeric compound having the following structure where the DR1 structure is bound to the side chain of PMMA has been developed.

The glass-transition temperature of this nonlinear-optically active polymeric compound is about 165° C. and higher than the glass-transition temperature of PMMA (about 100° C.). In contrast to the fact that DR1 can be dispersed in PMMA only at a concentration of up to 30% by mass without crystal precipitation, the nonlinear-optically active polymeric compound, which contains the DR1 structure at a concentration equivalent to 82% by mass, provides a clear film without phase separation. Accordingly, the nonlinear-optically active polymeric compound exhibits higher nonlinear-optical characteristics and higher stability than the dispersion-system compounds where DR1 is dispersed in PMMA.
However, even if polymerization is possible, it is difficult to both polymerize the monomers having a bulky nonlinear-optically active structure and control the degree of polymerization. If the degree of polymerization is not raised sufficiently, the resulting polymers have significantly lower mechanical strength. In addition, insufficient control of the degree of polymerization causes a problem in that it is difficult to produce films having a certain consistent thickness due to fluctuation by production lot of the viscosity of coating solutions (stock solutions). As purification of polymers is generally difficult, residual impurities such as polymerization catalysts may also make it more difficult to apply an effective electric field during electric-field poling. Therefore, introduction of a nonlinear-optically active organic compound into the main chain and/or side-chain of a polymer is hardly the best method.
To solve the problems associated with the aforementioned dispersion-type organic nonlinear-optical materials, a method of preparing a cross-linkable nonlinear-optically active organic compound is being studied. A nonlinear-optically active organic compound such as DR1 is introduced to the cross-linkable functional group in the compound, and the cross-linkable nonlinear-optically active organic compound is coated and dried, after which electric-field poling and curing by cross-linking treatments are conducted simultaneously (hereinafter, referred to as a “curing by cross-linking system”). This method provides a favorable effect of stabilizing the oriented state significantly, as it fixes the oriented state induced by the electric-field poling by cross-linking. In addition, since the raw material, i.e., the cross-linkable nonlinear-optically active organic compound, is a low-molecular weight compound, the problems concerning the polymerization and purification of the aforementioned nonlinear-optically active polymeric compound are reduced.
However, conventional nonlinear-optically active organic compounds are highly aggregative, and thus even if cross-linking and curing of organic compounds having a cross-linkable functional group is conducted, such organic compounds tend to aggregate or crystallize in the drying step prior to curing by cross-linking and clear cured films cannot be obtained. These compounds are also problematic in that a cross-linking reaction causes gelation of the stock solution, or with pot life of the stock solution due to precipitation, leading to deterioration in optical quality and increase in the production cost of the resulting films.